Nitride-based multi-junction solar cell modules and methods for making the same

ABSTRACT

A backside illuminated multi junction solar cell module includes a substrate, multiple multi junction solar cells, and a cell interconnection that provides a series connection between at least two of the multi junction solar cells. The substrate may include a material that is substantially transparent to solar radiation. Each multi junction solar cell includes a first active cell, grown over the substrate, for absorbing a first portion of the solar radiation for conversion into electrical energy and a second active cell, grown over the first active cell, for absorbing a second portion of the solar radiation for conversion into electrical energy. At least one of the first and second active cells includes a nitride.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of and claims the benefit of co-pendingU.S. patent application Ser. No. 12/031,338, filed on Feb. 14, 2008, andentitled “Nitride-Based Multi-Junction Solar Cell Modules and Methodsfor Making the Same,” which claims priority to and the benefit of U.S.provisional patent Application No. 60/922,484, filed on Apr. 9, 2007,and entitled “Nitride-Based Multi-Junction Solar Cell Modules andMethods for Making the Same,” which disclosures are their entireties.

TECHNICAL FIELD

The invention generally relates to solar cells. More particularly, theinvention relates to III-nitride-material based monolithic multijunction (MJ) solar cells, their related photovoltaic devices, andmethods for making the same.

BACKGROUND

Solar photovoltaic devices (i.e., solar cells) are devices capable ofconverting solar radiation into usable electrical energy. Commonly usedsemiconductor solar cell devices are typically composed of one or morepairs of p-n junction cells, which include a p-type semiconductor layeradjacent an n-type semiconductor layer. Energy conversion occurs assolar radiation impinging on the solar cell and absorbed by an activeregion of semiconductor material generates electricity. If properlydesigned, multi junction solar cells may be more efficient than singlejunction solar cells, because a larger portion of the solar spectrum canbe captured.

In order for the solar cell device to be economical and highlyefficient, there must be an availability of high quality semiconductormaterials, a flexible choice of junction band-gaps covering a broadsolar spectrum, and an appropriate device architecture design thatmaximizes current match and minimizes electrical/optical losses. Inaddition, the solar cell device should minimize environmental pollutionand manufacturing cost. To date, high-efficiency III-V semiconductormulti junction solar cells have typically been grown on GaAs, InP, andGe substrates using GaInP, and (In)GaAs cell structures to absorb solarradiation energy between 0.7 eV and 1.8 eV. Several such designs aredescribed in U.S. Pat. Nos. 5,223,043; 5,405,453; and 5,407,491.However, significant fractions of solar radiation at wavelengths longerthan 900 nm and shorter than 700 nm generally have not been effectivelyused due to material band gap limits in existing solar cells.

Environmental hazards are another issue with existing solar cells, suchas with conventional III-V solar cell devices composed of GaAs andInGaP, which are environmentally hazardous elements after materialdecomposition. Also, the cost of using substrates such as GaAs and Ge ishigh.

SUMMARY OF THE INVENTION

The present invention realizes full-solar-spectrum, high-efficiency,robust, and low cost solar cells and photovoltaic (PV) devices usingIII-nitride semiconductor compounds. Because the energy band gap of aIII-nitride material system can be engineered from 0.7 eV to 6.0 eV,III-nitride-based solar cells can absorb solar energy from a much widerspectrum. In addition, as compared with conventional solar cellmanufacturing techniques, major environmental pollution issues relatedto material growth, device fabrication, material handling, and disposalare avoided by using III-nitride semiconductor compounds. TheIII-nitride-based multi junction solar cells of the present inventionpossess many useful advantages in, for example, space applications, suchas satellites, spacecraft, or space stations, and terrestrialapplications that can benefit from low maintenance and long lastingsources of solar-generated electricity, such as facilities remote frommain electrical grids or outback telecommunications stations.

In one embodiment of the invention, a monolithic multi junction solarcell device includes III-nitride alloys grown on a sapphire substrate.This solar cell device, in contrast to conventional III-V solar celldevices, may exhibit the following features and advantages. First,III-nitride materials (such as GaN, InN, AlN, or their ternary orquaternary alloys) may be used to construct the tandem solar cell on adouble-side polished solar-transparent substrate, such as sapphire. Thefull cell structure may be monolithically grown using appropriatethin-film deposition techniques such as metal-organic chemical vapordeposition (MOCVD), molecular beam epitaxy (MBE), or other appropriateprocesses. Second, a monolithic interconnected module (MIM) may be usedto form internal circuit connections. The size of the MIM array is onlylimited by the size of the substrate upon which growth takes place. Thisenables fabricating larger-area solar cell arrays. Third, thefabrication of the MIM can be accomplished using standard industrialphotolithographic processes, which permits changes in circuit design bysimply altering the photomask pattern and device architecture. Fourth, adouble-side polished substrate may be used as a solar cell cover sheetby flip-chip wafer bonding. Such substrates offer many advantages overconventional glass sheet materials, including a higher radiation damagethreshold, a maximal effective solar absorption area, and a higherbroad-band solar transmission efficiency.

In general, in one aspect, the invention features a backside illuminatedmulti junction solar cell module. The solar cell module includes asubstrate, multiple multi junction solar cells, and a cellinterconnection that provides a series connection between at least twoof the multi-junction solar cells. The substrate includes a materialsubstantially transparent to solar radiation. Each multi junction solarcell includes a first active cell, grown over the substrate, forabsorbing a first portion of the solar radiation for conversion intoelectrical energy and a second active cell, grown over the first activecell, for absorbing a second portion of the solar radiation forconversion into electrical energy. At least one of the first and secondactive cells includes a nitride.

In general, in another aspect, the invention features a method formaking a monolithic interconnected module. The method includes forming aplurality of solar cell mesas over a substrate, providing on each mesa afirst active cell for absorbing solar radiation for conversion intoelectrical energy, providing over each first active cell a second activecell for absorbing solar radiation for conversion into electricalenergy, and electrically connecting the plurality of solar cell mesas.The plurality of solar cell mesas may be electrically connected inseries or in parallel. Alternatively, a first portion of the pluralityof solar cell mesas may be electrically connected in series and a secondportion electrically connected in parallel. At least one of the firstand second active cells of each mesa includes a nitride.

In various embodiments of these aspects of the invention, both the firstand second active cells of at least one multi junction solar cell ormesa include a nitride, such as a III-nitride material. For example, thefirst active cell may include gallium nitride and the second active cellmay include indium gallium nitride. At least one of the first and secondactive cells may also include a ternary or quaternary alloy. The firstactive cell may be grown to absorb solar energy between approximately3.4 electrovolts and approximately 4.0 electrovolts, while the secondactive cell may be grown to absorb solar energy between approximately2.0 electrovolts and approximately 3.4 electrovolts. An interconnectingtunnel junction may be provided between the first and second activecells of at least one multi junction solar cell or mesa. Theinterconnecting tunnel junction may facilitate the flow ofphotogenerated electrical current between the first and second activecells and may include a nitride, such as, for example, gallium nitride.

In another embodiment, a third active cell for absorbing solar radiationfor conversion into electrical energy is grown over the second activecell of at least one multi junction solar cell or mesa. The third activecell may include a nitride, such as, for example, indium nitride, andmay be grown to absorb solar energy between approximately 0.7electrovolts and approximately 2.0 electrovolts. An interconnectingtunnel junction may be provided between the second and third activecells of at least one multi junction solar cell or mesa. Theinterconnecting tunnel junction may facilitate the flow ofphotogenerated electrical current between the second and third activecells and may include a nitride, such as, for example, indium galliumnitride.

In the above embodiments, the second active cell of at least one multijunction solar cell or mesa may be grown to absorb a narrower band ofsolar energy than its first active cell. Similarly, the third activecell of at least one multi junction solar cell or mesa may be grown toabsorb a narrower band of solar energy than both its first and secondactive cells. The substrate over which the first active cells may begrown or disposed may include a material that is substantiallytransparent to solar radiation and/or a material that is electricallyunconductive. For example, the substrate may include sapphire.

A dielectric thin film and/or a contact grid may be deposited on atleast one of the multi junction solar cells or mesas prior toelectrically connecting the solar cells or mesas. The plurality of multijunction solar cells or mesas may also be bonded to an electricallyisolated carrier, such as, for example, a glass plate.

In general, in yet another aspect, the invention features a method formaking a backside illuminated monolithic interconnected module. Themethod includes providing a transparent substrate having a top andbottom, providing an array of solar cells above the top of thesubstrate, and positioning the bottom of the substrate to face aradiation source.

In various embodiments of this aspect of the invention, the substrateincludes or consists essentially of a sapphire material. At least one ofthe solar cells may be a multi junction solar cell and may include aIII-nitride material.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more apparent and may be better understood byreferring to the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a sectional view of one embodiment of a three junction solarcell device structure;

FIG. 2 is a sectional view of one embodiment of a MIM solar cellconstructed in accordance with the present invention; and

FIG. 3 is a sectional view of one embodiment of a flip-chip waferbonding scheme.

DESCRIPTION

In general, the present invention pertains to III-nitride-material basedmonolithic multi junction solar cells, their related photovoltaicdevices, and methods for making the same.

FIG. 1 depicts a sectional view of an exemplary embodiment of a threejunction solar cell device structure 10 grown on a substrate 20. Asshown in the illustrative embodiment, the solar cell device 10 mayinclude, in the order of growth over the substrate 20, a buffer 30, afirst cell 40, a first interconnecting tunnel junction 50, a second cell60, a second interconnecting tunnel junction 70, a third cell 80, and acap layer 90. Each of the first, second, and third cells 40, 60, 80 maybe a p-n junction cell.

In one embodiment, the substrate 20 includes an optically transparentand electrically insulating material having high opticalsolar-transmission efficiency from the UV to infrared wavelength region.The substrate 20 may be, for example, a double-side polishedsingle-crystal sapphire substrate or a semi-insulating SiC substrate.The buffer layer 30, which is formed over the substrate 20, may includea III-nitride material, such as GaN, AlN, or their alloys.

In the embodiment of FIG. 1 (and also in the embodiments of FIGS. 2-3,discussed below), epitaxial formation of the three junction solar celldevice 10 may include sequentially growing active p-n junctions withwider band-gap semiconductor materials prior to growing junctions withnarrower band gap materials. Using this growth sequence reduces possiblematerial heterointerface-diffusion during epitaxy, because the widerbandgap III-nitride material typically requires a higher growthtemperature.

In one embodiment, the first p-n junction cell 40 is used for absorbingsolar energy in a range from approximately 3.4 eV to approximately 4.0eV and is grown on the buffer layer 30. The first cell 40 may include atleast one n-type layer and one p-type layer, such as an n-type (orp-type) base layer 41 and a p-type (or n-type) emitter layer 42. Thebase layer 41 and the emitter layer 42 may include or consistessentially of, for example, III-V materials, such as GaN—GaN orAlGaN—AlGaN homojunction layers or their heterostructural GaN—AlGaNformat.

The first tunnel junction 50 may be formed to facilitate the flow ofphotogenerated electrical current between the first cell 40 and thesecond cell 60. The first tunnel junction 50 may take a number of formsto provide a thin layer of material (usually the same material as theemitter layer 42 of the first cell 40 or as an overlying base layer 61of the second cell 60) that allows current to pass between the first andsecond cells 40, 60 without generating a voltage drop large enough tosignificantly decrease the conversion efficiency of the device 10. Forexample, as illustrated in FIG. 1, each of the layers 51, 52 of thefirst tunnel junction 50 may include or consist essentially of GaN.

In one embodiment, the second cell 60 is used for absorbing solar energyin a range from approximately 2.0 eV to approximately 3.4 eV. The secondcell 60 may include at least an n-type (or p-type) base layer 61 and ap-type (or n-type) emitter layer 62. The material for the second cell 60may include or consist essentially of InGaN, with constant indiumcomposition or graded indium content.

The second tunnel junction 70 may be used to facilitate the flow ofphotogenerated electrical current between second cell 60 and the thirdcell 80. The second tunnel junction 70 may take any of a number of formsto provide a thin layer of material (usually the same material as eitherthe emitter layer 62 of the second cell 60 or the base layer 81 of theoverlying third cell 80) that allows current to pass between the secondcell 60 and the third cell 80 without generating a voltage drop largeenough to significantly decrease the conversion efficiency of the device10. For example, as illustrated in FIG. 1, each of the layers 71, 72 ofthe second tunnel junction 70 may include or consist essentially ofInGaN.

For the illustrated three junction version of the solar cell device 10,the third cell 80 is the last cell, covering optical absorption in therange of approximately 0.7 eV to approximately 2.0 eV. The third cell 80may include at least an n-type (or p-type) base layer 81 and a p-type(or n-type) emitter layer 82. The material for either the base layer 81or the emitter layer 82 may include or consist essentially of InN withconstant indium composition or graded indium content. In the illustratedembodiment, the cap layer 90 is the final deposited layer for makinginternal electrical contact for an MIM module, and may include orconsist essentially of, for example, InN, GaN, or InGaN. As analternative to III-nitride materials, diluted nitride may be used asappropriate, for example in base layer 81 and emitter layer 82 of thethird cell 80.

Solar cell device 10 may be formed by any suitable epitaxial depositionsystem or combination of systems, including, but not limited to,metal-organic chemical vapor deposition (MOCVD), atmospheric-pressureCVD (APCVD), low- (or reduced-) pressure CVD (LPCVD), ultra-high-vacuumCVD (UHCVD), molecular beam epitaxy (MBE), or atomic layer deposition(ALD). In the CVD process, exemplary source materials would includetrimethylgallium (TMG), triethylgallium (TEG), trimethylaluminum (TMA),trimethylindium (TMI), ammonia, or dimethylhydrozane (DMHy). The carriergas may be, for example, hydrogen or nitrogen. The selection of variousprecursors and the utilization of different growth methods is understoodamong those skilled in the art.

In an exemplary process, triple junction crystalline material 10 isgrown using MOCVD. In accordance with that process, the double-sidepolished-sapphire substrate 20 is first thermally annealed with hydrogenat approximately 1100° C. for 10 minutes with chamber pressure ofapproximately 50 torr. Then, the temperature is cooled down toapproximately 530° C. and the chamber pressure is ramped up toapproximately 500 ton for the growth of the buffer layer 30. NH₃pre-exposure is conducted by flowing NH₃ gas through the reactor duringtemperature ramp down. An approximately 30-50 nm thick buffer layer 30is then grown by introducing TEG into the reactor at approximately 530°C. The thickness and growth optimization of the buffer layer 30 arecontrolled by in-situ monitoring of the nucleation process on thesurface of the sapphire substrate 20.

After growth of the buffer layer 30, the chamber temperature is rampedup to approximately 1050° C. with only NH₃ flowing through the reactor.The first GaN cell 40 as shown in FIG. 1 is then grown at approximately1050° C. using TMG as the precursor. A SiH₄ doped 2 μm GaN base layer 41is grown first, followed by a 1 μm Cp₂Mg doped emitter layer 42. Thedoping concentration is approximately in the range of 1-5×10¹⁷ cm⁻³ forboth layers 41, 42.

The first interconnecting tunnel junction 50 is then grown, preferablyunder the same conditions as for the first cell 40, with a 50 nm highlydoped n-layer 51 and a 50 nm highly doped p-layer 52. Then, the carriergas is switched from hydrogen to nitrogen and the temperature isdecreased to approximately 850° C. An in-situ thermal annealing atapproximately 850° C. for 5 minutes is performed with N₂ to activatep-type carriers in pre-grown layers 42 and 52. After annealing, thechamber pressure is increased from approximately 500 torr toapproximately 600 torr and the chamber temperature is decreased toapproximately 800° C.

The second cell 60 is then grown at approximately 800° C. using TEG andTMI as the precursors. SiH₄ is used for n-type doping in the 0.1 μm baselayer 61 and Cp₂Mg is used for p-type doping in the 1 μm emitter layer62. The doping concentration is in the range of approximately 1-3×10¹⁷cm⁻³ for both layers. A 50 nm highly doped n-layer 71 and a 50 nm highlydoped p-layer 72 are then grown to form the second tunnel junction 70under the same growth conditions as for the second cell 60. A secondthermal annealing is conducted after growing the second interconnectingtunnel junction 70 for 5 minutes in N₂ ambient at a temperature ofapproximately 750° C.

The growth temperature is then further decreased to approximately 680°C. for growing the third cell 80. In this exemplary embodiment, thethird cell 80 includes a 50 nm n-type base layer 81 having anapproximate doping concentration of 1-5×10¹⁹ cm⁻³ and a 100 nm p-typeemitter layer 82 having an approximate doping concentration of 1×10¹⁷cm⁻³. Then, a 100 nm p-type cap layer 90 is grown. A third thermalannealing is conducted after growing layer 90 for 5 minutes in N₂ambient at a temperature of approximately 650° C.

FIG. 2 depicts a sectional view of one embodiment of a MIM solar cellconstructed in accordance with the present invention. In one embodiment,after growth of a device 10, a pair of 1×1 cm² solar cell mesas 145 areformed by etching a first trench 135 that extends through the solar celldevice 10 from the cap layer 90 partially into the base layer 41 of thefirst cell 40. The first trench 135 may have a width d₂ of, for example,approximately 10 μm, although other suitable widths may also be used.Moreover, more than one such first trench 135 may be etched. Where morethan one such first trench 135 is etched, the distance d₁ between thefirst trenches 135 may be approximately 100 μm, although other suitabledistances may also be used.

Then, the remaining portion of the base layer 41 of the first cell 40and the buffer layer 30 inside the first etched region or trench 135 mayboth be partially etched out to form a second trench 140 and to isolateindividual solar cell mesas 145. The width d₃ of the second etchedtrench 140 may be approximately 2 μm, although other suitable widths mayalso be used. A dielectric thin film 100, formed from, for example, SiO₂or SiN_(x), may then be deposited over the wafer in order to reducecurrent leakage on etched mesa surfaces. This highly reflectivepassivating film may also function as an optical reflector as describedbelow. Then, 5 μm n-contact grids 130 may be formed by, for example,depositing multiplayer metals such as Al/Ni on the exposed n-GaNtemplate of the base layer 41 of the first cell 40. In addition, 20 μmmetal grids 120 may be formed on p-type cap layer 90 by depositingAu/Ti. Internal series connections 110 between individual solar cellmesas 145 are schematically illustrated in FIG. 2. The connections 110may be made from Au, or other suitable alloys such as Au/Sn.

Because, as illustrated in FIG. 2, individual solar cell mesas 145 areconnected in series, voltage may build up across the solar cell mesas145 while current remains constant. This can lead to smaller powerlosses for a given area device. For this reason, the MIM itself maybecome dimensionally large without any outside interconnections. Thisprovides potential advantages in practical applications requiring largepanel assemblies.

Alternatively, the individual solar cell mesas 145 may be connected inparallel or, in yet another embodiment, a first portion of theindividual solar cell mesas 145 may be connected in series and a secondportion may be connected in parallel. Moreover, as will be understood byone skilled in the art, any number of solar cell devices 10 and/or solarcell mesas 145 may be constructed on the substrate 20. In other words,an array of solar cell devices 10 and/or solar cells mesas 145 may bepresent on the substrate 20.

FIG. 3 depicts a sectional view of one embodiment of a flip-chip waferbonding scheme. As shown, the MIM solar cell of FIG. 2 has been rotatedby 180° so that the bottom surface of the substrate 20 faces upwards andin the direction of a solar radiation source (not shown). Thus, the MIMsolar cell is backside illuminated, in the sense that the light from theradiation source enters the bottom surface of the substrate 20 andpropagates through the MIM solar cell towards an electrically isolatedwafer carrier 150. Because crystal sapphire material has opticalcharacteristics superior to those of standard glass materials, with upto 98.5% transmission and an extremely wide transmission bandwidth from190 nm to 5 microns, the double-side polished sapphire substrate 20 maybe used as the solar cell cover sheet by flip-over wafer bonding. Inaddition, sapphire's superior radiation-resistance makes it an excellentmaterial for space applications.

Device packaging may be completed by bonding MIM cell arrays with theelectrically isolated wafer carrier 150, such as a glass plate, as shownin FIG. 3. Before bonding, 8-10 μm thick indium grids 160 may bedeposited on pre-patterned carrier 150 so that grids 160 match the gridpattern of the MIM array. A dielectric layer 170, formed from, forexample, SiO₂, may be deposited in between the indium grids to providean optical reflection mirror for photon recycling. Wafer bonding can beperformed by various processes as understood by those skilled in theart, for example using a commercial flip-chip bond machine or by othermeans, such as by manual operation with an appropriate opticalmicroscope. In the illustrated embodiment, the bonding process may becompleted through the applied pressure due to the low (156° C.) meltingpoint of indium.

Solar cell embodiments constructed in accordance with the techniquesdiscussed above can provide a higher photovoltaic efficiency than solarcells based on the use of amorphous silicon on silicon substrates, andcan be constructed at a cost lower than for solar cells that are basedon the use of III-V materials on substrates such as Ge or GaAs. Inaddition, the back-side illumination feature of an optically transparentsubstrate such as sapphire, which is desirably used as the interfacebetween the solar cell and the light source, provides advantages overthe conventional use of glass because sapphire is harder than glass, canstand up to heat better than glass, and can resist or block particlessuch as gamma rays and protons better than glass.

Having described certain embodiments of the invention, it will beapparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. For example, whilethe present invention has been described with reference to a threejunction solar cell device structure 10, a person skilled in the artwill understand that other embodiments different from device 10, forexample one-, two-, four-, or more junction solar cell devicestructures, are within the scope of the present invention. In addition,all measurements (e.g., distances, widths, etc.), temperatures,pressures, and time frames mentioned herein to describe the inventivedevices and methods of manufacture are approximate (even if notindicated as such) and may be varied slightly to suit a particularapplication, as will be understood by one of skill in the art.Accordingly, the described embodiments are to be considered in allrespects as only illustrative and not restrictive.

What is claimed is:
 1. A method comprising: forming a plurality of solarcell mesas over a substrate; providing, in each mesa, a first activecell for absorbing solar radiation for conversion into electricalenergy, the providing the first active cell comprising epitaxiallygrowing a first active cell layer over the substrate, the first activecell layer comprising a first p-n junction, the first active cell havinga first bandgap energy; providing, in each mesa and over each firstactive cell, a second active cell for absorbing solar radiation forconversion into electrical energy, the providing the second active cellcomprising epitaxially growing a second active cell layer over the firstactive cell layer, the second active cell layer comprising a second p-njunction, the second active cell having a second bandgap energy, thefirst bandgap energy being greater than the second bandgap energy;forming a dielectric layer over the plurality of solar cell mesas andthe substrate, the plurality of solar cell mesas being disposed betweenthe substrate and at least a portion of the dielectric layer; andelectrically connecting the plurality of solar cell mesas, wherein atleast one of the first and second active cells of each mesa comprises anitride.
 2. The method of claim 1, wherein the substrate comprises amaterial that is substantially transparent to solar radiation.
 3. Themethod of claim 1, wherein the substrate comprises silicon, and at leastone of the first and second active cells of at least one mesa comprisesa III-nitride material.
 4. The method of claim 1, wherein the firstactive cell absorbs solar radiation in a range from approximately 3.4 eVto approximately 4.0 eV, and the second active cell absorbs solarradiation in the range from approximately 2.0 eV to approximately 3.4eV.
 5. The method of claim 4 further comprising providing, in each mesaand over each second active cell, a third active cell for absorbingsolar radiation for conversion into electrical energy, the third activecell absorbing solar radiation in a range from approximately 0.7 eV toapproximately 2.0 eV.
 6. The method of claim 1 further comprisingbonding the plurality of solar cell mesas to a carrier, the plurality ofsolar cell mesas being disposed between the substrate and the carrier.7. The method of claim 1 wherein the substrate comprises an electricallyisolating portion.
 8. The method of claim 1 further comprising forming abuffer layer over the substrate, the buffer layer being disposed betweenthe substrate and the first active cell of each mesa.
 9. The method ofclaim 1 further comprising providing, in each mesa and over each secondactive cell, a third active cell for absorbing solar radiation forconversion into electrical energy, the third active cell having a thirdbandgap energy, the second bandgap energy being greater than the thirdbandgap energy.
 10. The method of claim 1, wherein the epitaxiallygrowing the first active cell includes using a process at a firsttemperature, the epitaxially growing the second active cell includesusing a process at a second temperature, and the first temperature isgreater than the second temperature.
 11. The method of claim 1 furthercomprising providing, in each mesa and over each second active cell, athird active cell for absorbing solar radiation for conversion intoelectrical energy, the third active cell having a third bandgap energy,the second bandgap energy being greater than the third bandgap energy,wherein the forming the plurality of solar cell mesas, the providing thefirst active cell, the providing the second active cell, and theproviding the third active cell further comprises: epitaxially growing athird active cell layer over the second active cell, the third activecell comprising a third p-n junction.
 12. The method of claim 11,wherein the epitaxially growing the first active cell includes using aprocess at a first temperature, the epitaxially growing the secondactive cell includes using a process at a second temperature, theepitaxially growing the third active cell using a process at a thirdtemperature and the first temperature is greater than the secondtemperature, the second temperature being greater than the thirdtemperature.
 13. The method of claim 1 further comprising etching thefirst active cell layer and the second active cell layer to provide theplurality of solar cell mesas.
 14. The method of claim 1 furthercomprising electrically connecting the plurality of solar cell mesas inseries.
 15. The method of claim 1 further comprising: forming a firstcontact grid through the dielectric layer to a base portion of eachmesa; and forming a second contact grid through the dielectric layer toa distal portion of each mesa, the distal portion of each mesa beingdistal from the substrate.
 16. The method of claim 1 further comprisingelectrically connecting the plurality of solar cell mesas in parallel.17. The method of claim 1, wherein the dielectric layer is an opticallyreflective dielectric layer.
 18. The method of claim 1 furthercomprising: forming a grid on a carrier substrate that corresponds to apattern of the plurality of solar cell mesas; and bonding the pluralityof solar cell mesas to the grid on the carrier substrate.
 19. The methodof claim 18 further comprising forming an optically reflectivedielectric layer on the carrier substrate and within the grid.